The demand for new and improved electronic devices such as cellular phones, notebook computers and compact camcorders have demanded energy storage devices having increasingly higher specific energy densities. A number of advanced battery technologies have recently been developed to service these devices, such as metal hydride (e.g., Ni—MH) and lithium batteries with liquid electrolytes. More recently, lithium batteries with polymer electrolytes have been introduced.
Lithium batteries can be either lithium ion batteries or lithium metal batteries. Lithium ion batteries intercalate lithium ions in a host material, such as graphite, to form the anode. On the other hand, lithium metal batteries use metallic lithium for the anode.
The electrolyte used in lithium batteries can be a liquid or a polymer electrolyte. Lithium batteries having liquid electrolytes have been on the market for several years. Lithium batteries having polymer electrolytes are comparatively new entries into the marketplace.
Lithium batteries having polymer electrolytes represent an evolving alternative to lithium batteries having liquid electrolytes. The polymer electrolyte provides a medium for ionic conduction analogous to the liquid electrolyte in a Li battery having a liquid electrolyte. Typical polymer electrolytes include polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA) and polyvinylidene fluoride (PVDF).
Because its electrolyte is a non-volatile material which cannot leak, a lithium battery having a polymer electrolyte is intrinsically safer that a lithium battery having a liquid electrolyte. Moreover, polymer electrolytes eliminate the need for venting and package pressure which are generally required for operation of lithium batteries having liquid electrolytes. Thus, polymer electrolytes make it possible to use a soft outer case such as a metal plastic laminate bag, resulting in improvement in weight and thickness, when compared to liquid electrolyte can-type Li batteries. In addition, recent research has indicated that electrode materials generally react less with polymer electrolytes compared to liquid electrolytes, even under abuse conditions. This should constitute a significant safety advantage for stable charging-discharging of Li batteries having polymer electrolytes over conventional Li batteries having liquid electrolytes. Lithium batteries having solid polymer electrolytes are also considered environmentally acceptable.
Lithium batteries having polymer electrolytes are generally configured as gel-type or plasticized polymer electrolytes which have liquid electrolytes intermixed with a selected polymer electrolyte matrix material. The liquid electrolyte generally includes a salt of an alkali metal and an aprotic solvent. The salt and the aprotic solvent comprising the liquid electrolyte are integrated with the polymer material to form the gel-type or plasticized polymer material.
The polymer electrolyte functions as a separator, being interposed between the cathode and anode films of the battery. Each cathode, separator and anode combination forms a unit battery cell. Practical lithium batteries, such as those having polymer electrolytes, are generally prepared by stacking a number of battery cells in series to achieve desired battery capacity.
Many performance parameters of lithium batteries are associated with the electrolyte choice, and the interaction of the selected electrolyte with the cathode and anode materials used. High electrolyte ionic conductivity generally results in improved battery performance. The ionic conductivity of polymer electrolytes has been reported to as high as approximately 10−4 S/cm. However, it is desirable for the ionic conductivity of the polymer electrolyte to reach a value of at least approximately 10−3 S/cm for many battery applications. In addition, it would also be desirable to enhance the electrochemical stability of the polymer electrolyte towards anode and cathode materials to improve battery reliability, as well as storage and cycling characteristics.
The ionic conductivity of a gel or plasticized polymer electrolyte can be significantly influenced by the specific alkali metal salt and aprotic solvent used. In polymer electrolytes, inorganic lithium salts, such as LiClO4, LiAsF6, LiPF6 and LiBF4, are widely used as traditional ion-conducting additives.
The lithium salt can influence the performance of rechargeable lithium-polymer power sources by affecting the transport of lithium ions in the volume of the polymer electrolyte and at the electrode/electrolyte interface or the formation of passivating layers at the electrode/polymer electrolyte interface, which are known to generally change during charge/discharge cycling.
In plasticized and gel-like polymer electrolytes containing as a plasticizer an aprotic solvent, cations and anions derived from lithium salts are each mobile. As a rule, the transport number of lithium ions in these systems is far less than 1 (usually ˜0.5). Such a situation can result from cation binding by the polar groups of polymer matrix, such as —O— in polyethylene oxide, —CN in polyacrylonitrile and—NR in polyamide. Therefore, in certain electrolytes including lithium salts, the anion can contribute to the overall ionic conductivity more than the cation can.
The problems of undesirable participation of anions in ion transport and decreasing reactivity of polymer electrolytes relative to the electrode materials can be solved by using lithium organic salts with large anions. It is evidenced by the increased interest in lithium imide salts of the LiN(SO3CF3)2, [(CF3SO2)(C4F9SO2)]NLi, (C4F9SO2)2NLi, types [J. Kuratomi, T. Iguchi, T. Bando, Y. Aihara, T. Ono, K. Kuwana. Development of Solid-Polymer Lithium Secondary Batteries//10th International meeting on Lithium Batteries “Lithium 2000”, Villa Erba Conference Center, Como, Italy, May 28-Jun. 2, Abstr.373] in the formation of polymer electrolytes.
Charge delocalization ability by the anion part of the salt resulting in their electrochemical and thermal stability is a characteristics property of these salts [J. Barthel, R. Buestrich, H. J. Gores, M. Schmidt, M. Wėhr, J.Electrochem.Soc, Vol. 144, N 11. 1997. P.3866-3870].
In the case of organic anions, these properties are known to depend directly on the acidity value of the corresponding organic acid. Salts which produce relatively strong NH-acid values can provide gel polymer electrolytes with high ionic conductivity. Some salts can also act as stabilizers with respect to thermo-oxidative destruction of polymers.
This is especially true for polymer electrolytes based on halogen-comprising polymers, for example, on the basis of polyvinylchloride (PVC) and its derivatives. Lithium salts like LiClO4, LiAsF6, LiPF6, LiBF4 are know to cause a thermal destruction of polymer electrolytes based on chlorinated polyvinyl chloride at above 50° C. [E. M. Shembel, O. V. Chervakov et al. Investigation of the stability of chlorinated PVC-based polymer electrolyte for lithium power sources//Journal Power Sources, 96 (2001) 20-28]. This limits the use of such polymer electrolytes in the composition of lithium power sources with operating temperature ranging from about −30 to +60° C.